98 bravada, already redid much of the front suspension.

i get a vibration at highway speeds, felt "in the seat" mostly. starts at around 60 mph and gets fairly noticeable around 70mph. i assumed it was a wheel balance issue with one of the rear tires/wheels, but the problem is, it comes and goes, and as well, unlike other balance issues i've had, it gets much, much worse with speed. it feels like the whole truck is going to fly apart at 80-90 mph.

the driveshaft u-joints don't seem to have any play, and the driveshaft doesn't look like it's missing balancing weights.

anything else i should check?

 

If your seat is vibrating..It maybe exhaust rubbing somewhere.Or a bad motor mount

 

Did you pull the driveshaft and move the U-Joints through their full range of motion? Should feel totally smooth, no tight spots or roughness. I have seen many a U-Joint that had no play in it, but was shot because the needle bearings had worn grooves into the bearing journals on the cross. They can also lock up solid. Also check your pinion bearings for movement, Chock the wheels, put it in neutral, and shake the yokes on the diffs, there should be no movement. Check the slip yoke going into the transfer case too, a little movement is normal, a lot is a problem. If the front driveshaft is a 2 piece style, try to wiggle the section where it joins, should be very little movement. If it has the CV joint at the rear, that joint could be rough causing the vibration.


Yours should have an All Wheel Drive system, check to make sure all 4 tires are the same circumference. Easy way to do this is park on a flat parking lot and place a piece of masking tape on each tire sidewall pointing straight down. Have someone watch the tires as you slowly pull STRAIGHT ahead, going two complete revolutions of the tires. Have them signal you to stop when one of the tapes is again vertical. ALL the tapes should be within a degree or two of vertical, if one or more is out of place the AWD system may be engaging.


Check for a broken belt in the tires too, have someone slowly drive the vehicle straight while you look at the tread rolling along. If it has a spot where it appears to wobble, you found your vibration.